1. Field of the Invention
The present invention relates generally to a modulation/demodulation scheme in an Orthogonal Frequency Division Multiple Access (OFDMA) mobile communication system, and more particularly to a method for allocating subchannels in order to apply an optimum modulation/demodulation scheme according to channel conditions.
2. Description of the Related Art
Currently, mobile communication systems are being developed to provide high speed and high quality mobile communication services, such as data services and multimedia services beyond the initial voice-centered services. Further, the 3rd generation (3G) mobile communication systems, which is commonly classified into asynchronous mobile communication systems (3GPP) and synchronous mobile communication systems (3GPP2), are being standardized for high speed and high quality wireless data packet services. For example, the 3GPP is now preparing standardizations for the High Speed Downlink Packet Access (HSDPA) scheme and the 3GPP2 is now preparing standardizations for the 1× Evolved Data and Voice (1× EV-DV) scheme. Moreover, the 4th generation (4G) mobile communication systems are intended to provide higher speed and higher quality multimedia service than the 3G mobile communication systems.
However, due to the wireless channel conditions, various factors may disturb employment of high degree modulations and high coding rates, which are necessary to provide a high speed and high quality data service. Such factors include white noise, reception signal power changes due to fading, shadowing, Doppler effect due to movement and frequent velocity change of a terminal, interference by another user and multipath signals, etc. Therefore, it is necessary to use a proper modulation scheme and coding scheme in consideration of the changing wireless channel conditions.
In order to provide such a high speed and high quality data service, further advanced technology having an enhanced adaptability for the changing channel conditions as well as the conventional technologies provided by the existing mobile communication systems is necessary.
An Adaptive Modulation and Coding Scheme (AMCS) is a representative of proposals of such advanced technologies. In the AMCS, the modulation scheme and the coding scheme of a data channel are determined according to the channel conditions between a cell and a user, thereby improving the use efficiency of the entire cell. The AMCS classifies the channel conditions into multiple levels and adaptively determines the modulation scheme and the coding scheme according to the levels.
Quadrature Phase Shift Keying (QPSK), 8PSK, and 16QAM (Quadrature Amplitude Modulation) are currently discussed as the possible modulation/demodulation schemes. Additionally, various coding rates, e.g., from ¼ to 1, are considered as the coding rates for the AMCS. Therefore, a mobile communication system using the AMCS applies a high degree modulation/demodulation scheme (8PSK, 16QAM, etc.) and a high coding rate to terminals using high quality channels (e.g., terminals located adjacent to a base station). However, the mobile communication system using the AMCS applies a low degree modulation/demodulation scheme (QPSK) and a low coding rate to terminals using relatively low quality channels (e.g., terminals located at a cell boundary region).
Use of an Orthogonal Frequency Division Multiplexing (OFDM) scheme is highly recommended for the high speed and high quality data service as described above. Therefore, use of the OFDM scheme is now taken into deep consideration for the 4G mobile communication system.
A representative system utilizing the OFDM scheme is an OFDMA-Frequency Division Multiple Access (OFDM-FDMA) system utilizing a multiple access scheme for multiple users, in which all users simultaneously use the entire time while using other subchannels. In an OFDMA system utilizing the AMCS, effective assignment of subchannels to users and determination of modulation schemes for the users should be performed at each time period according to the channel conditions. This method can effectively use all subchannels, except when all users are in deep paging, because the channels between users in the same subchannel are independent from each other. Further, this method can provide a channel diversity effect between users at different locations.
Current research in the AMCS for a system utilizing the OFDMA scheme are being in looked at from two points of view. One standpoint relates to a Margin Adaptive (MA) problem for minimizing the entire transmission power based on given transmission rates of all users and the other standpoint relates to a Rate Adaptive (RA) problem for maximizing the transmission rates of users based on a given entire transmission power.
The RA problem for maximizing a transmission rate can be expressed as shown in Equation (1).
                                                        max                                                c                                      k                    ,                    n                                                  ,                                  ρ                                      k                    ,                    n                                                                        ⁢                                          min                k                            ⁢                              R                k                                              =                                    max                                                c                                      k                    ,                    n                                                  ,                                  ρ                                      k                    ,                    n                                                                        ⁢                                          min                k                            ⁢                                                ∑                                      n                    =                    1                                    N                                ⁢                                                                  ⁢                                                      c                                          k                      ,                      n                                                        ·                                      ρ                                          k                      ,                      n                                                                                                          ⁢                                  ⁢                                                                              subject                  ⁢                                                                          ⁢                  to                  ⁢                                     ⁢                                                            ∑                                              k                        =                        1                                            K                                        ⁢                                                                                  ⁢                                                                  ∑                                                  n                          =                          1                                                N                                            ⁢                                                                                                    f                            k                                                    ⁡                                                      (                                                          c                                                              k                                ,                                n                                                                                      )                                                                          ⁢                                                                              ρ                                                          k                              ,                              n                                                                                /                                                      α                                                          k                              ,                              n                                                        2                                                                                                                                              ≦                                  P                  T                                                                                                                        ⁢                                                                                                    ∑                                                  k                          =                          1                                                K                                            ⁢                                                                                          ⁢                                              ρ                                                  k                          ,                          n                                                                                      =                    1                                    ,                                      for                    ⁢                                                                                  ⁢                    all                    ⁢                                                                                  ⁢                    n                                                                                                          (        1        )            
The original problem expressed as Equation 1 can be replaced by Equation (2) as an equivalent expression shown below.
                                          max                                          c                                  k                  ,                  n                                            ,                              ρ                                  k                  ,                  n                                                              ⁢          z                ⁢                                  ⁢                                                                                                  subject                    ⁢                                                                                  ⁢                    to                    ⁢                                                                                  ⁢                                          R                      k                                                        =                                    ⁢                                                                                    ∑                                                  n                          =                          1                                                N                                            ⁢                                                                                          ⁢                                                                        c                                                      k                            ,                            n                                                                          ·                                                  ρ                                                      k                            ,                            n                                                                                                                =                    z                                                  ,                                  for                  ⁢                                                                          ⁢                  all                  ⁢                                                                          ⁢                  k                                                                                                                        ⁢                                                                            ∑                                              k                        =                        1                                            K                                        ⁢                                                                                  ⁢                                                                  ∑                                                  n                          =                          1                                                N                                            ⁢                                                                                          ⁢                                                                                                    f                            k                                                    ⁡                                                      (                                                          c                                                              k                                ,                                n                                                                                      )                                                                          ⁢                                                                              ρ                                                          k                              ,                              n                                                                                /                                                      α                                                          k                              ,                              n                                                        2                                                                                                                                ≤                                      P                    T                                                                                                                                          ⁢                                                                                                    ∑                                                  k                          =                          1                                                K                                            ⁢                                                                                          ⁢                                              ρ                                                  k                          ,                          n                                                                                      =                    1                                    ,                                      for                    ⁢                                                                                  ⁢                    all                    ⁢                                                                                  ⁢                    n                                                                                                          (        2        )            
In Equations (1) and (2), ρk,n is an identifier (binary variable) indicating that the k-th user occupies the n-th subcarrier, ck,n indicates the k-th user's bit allocated to the n-th subcarrier, Rk indicates k-th user's data rate, PT indicates total power, and ƒk(c) indicates required received power for reliable reception of c bits/symbol.
The AMCS for the system utilizing the OFDMA scheme independently considers the RA problem and the MA problem for minimizing the entire power with the fixed transmission rates that is, the AMCS researched to date cannot be used in an actual system including users of both standpoints, because the conventional AMCS independently considers the two standpoints. Therefore, in a system including some users to which services requiring fixed transmission rate such as Video-on-Demand (VOD) are provided, it is unnecessary to maximize the transmission rates of all users as opposed to the RA problem. Therefore, a new adaptive modulation technique is necessary in consideration of such a problem.
In order to satisfy such a necessity, an optimal solution for the adaptive modulation problem can be obtained by expressing the adaptive modulation problem as a nonlinear optimization problem and then converting the nonlinear optimization problem into a linear optimization problem that can be solved by Integer Programming (IP). However, such a method for obtaining an optimal solution cannot be used in real-time because of the complexity of the method.